1. Field of the Invention
This invention relates to the field of treating asphaltic concrete paving materials and the recycling of said materials. It is necessary to perform the recycling of the paving materials at the least cost per ton of material. The use of hot air or high heat combustion gases to heat asphaltic paving materials and the use of microwave fields is well known in the art.
The single greatest cause of inefficiency in heating asphaltic paving is the necessity for driving off water vapor. Until such water vapor is driven off and removed from the paving materials, the materials cannot be heated to their ideal working temperature which is well in excess of the boiling temperature of water.
In the prior art little concern has been given to efficient heat transfer, heat utilization, and to prevention of degradation of the asphalt while maintaining overall efficiency.
2. Brief Summary of the Invention
This invention is a method of treating a reclaimed asphaltic concrete paving material which utilizes as its first step the removal of loose fines (small particles)from the reclaimed asphalt paving (RAP). Strictly speaking, we are not removing all fines from the RAP. These are loose fines. There is a substantial portion of fines in the RAP which is bound up by the asphalt and contained in the larger agglomerates of RAP. These agglomerated fines are dry and are needed in the final mix. It has been found that the moisture in the RAP is on the surface of the particles of the material The small particles have a far greater surface area to volume ratio than the larger particles. The small particles have on their surfaces, the larger portion of the water in the RAP. Therefore, if the fine materials are removed prior to the heating of the RAP, a substantial savings in energy input may be achieved. It is also true that an excess of fines in the mix is unacceptable because it will not meet specifications for gradient of material.
In this invention, an air sorter and/or screens are used to separate out fine materials which are less than a predetermined size. The materials which are greater than the predetermined size (remaining RAP) are then first heated in a recirculating air dryer to a first predetermined temperature. The RAP at the first predetermined temperature is then heated to a higher predetermined temperature required for working the asphaltic pavement in a microwave tunnel. By the use of these three steps, substantial energy savings are achieved by first, eliminating substantial quantities of water through the removal of fines by the air sorter, secondly, by heating the RAP to a first predetermined temperature in a recirculating air dryer which is very efficient. Still further, efficiencies are achieved by the use of the microwave radiation tunnel which allows heating of the RAP without overheating the asphaltic components. Overheating causes smoking and inefficient use of the available heat and degradation of asphalt.
In another embodiment of this invention, the RAP is separated into a plurality of sizes, and each of the sizes is then heated in a separate conveying and heating apparatus. By separating into groups of different sizes, different heating rates or methods may be applied to each group, thereby eliminating the problem of overheating small particles and underheating larger particles. Through particle separation, better control over the asphaltic pavement recycling operation is achieved.
The microwave heater in accordance with this invention may comprise an aluminum or stainless steel enclosed microwave heating tunnel in combination with a stainless steel conveying belt. Still further, the microwave tunnel may provide for placement of the microwave antenna within three inches or less of the RAP which is being treated. Arcing and undesirable dust clouds can be kept to a minimum in the region of the microwave heating element by placement of a dust shield between the microwave antenna and the RAP passing through said tunnel. The dust shield may be fiberglass cloth or other suitable fabric which is transparent to the microwave.